SELMA, Ala. — Thousands of marchers, government officials and other public figures gathered Sunday for a second straight day to commemorate the 50th anniversary of a brutal police assault on civil rights demonstrators that spurred the passage of the Voting Rights Act.

Starting from early morning, groups of people — some with locked arms, some in song, some taking to their knees to pray — began to march across Selma’s Edmund Pettus Bridge, the site of Selma’s “Bloody Sunday” march on March 7, 1965. By midafternoon, police said at least 15,000 to 20,000 people had joined the crush on and around the bridge, according to the Associated Press

“Today we should be celebrating, but we can’t celebrate yet,” Martin Luther King III, the son of the Rev. Martin Luther King Jr., said at a memorial service Sunday at Brown Chapel African Methodist Episcopal Church.

“Our voting rights have been decimated,” King said.

Also in attendance was the outgoing U.S. attorney general, Eric H. Holder Jr., who criticized voting restrictions pursued by conservative lawmakers and what Holder called a “profoundly flawed” decision by the U.S. Supreme Court that weakened the federal government’s voting-rights enforcement powers.

“It has been clear in recent years that fair and free access to the franchise is still, in some areas, under siege,” Holder said at the memorial service. “Shortly after the historic election of President Obama in 2008, numerous states and jurisdictions attempted to impose rules and laws that had the effect of restricting Americans’ opportunities to vote — particularly, and disproportionately, communities of color.”

Jesse Jackson, one of many speakers at Brown Chapel on Sunday, called for a restoration of the Voting Rights Act and a renewed fight against poverty, which he called “a weapon of mass destruction.”

“Our struggle is not over,” Jackson said.

Holder urged state legislatures “to lift restrictions that currently disenfranchise millions of citizens convicted of felonies” and added that his expected successor, Loretta Lynch, who is also black, “will continue to fight aggressively on behalf of this sacred right.”

Holder also drew applause from the audience by making another nod to current controversies in Ferguson, Mo., and elsewhere across the United States. He noted that the activist Jimmie Lee Jackson, who was killed in Marion, Ala., in 1965, had been an unarmed black man.

“We will march on,” Holder said, calling on his listeners to “challenge entrenched power.”

Holder added that although he was leaving his post soon, “I want you to know that, no matter what I do or where my own journey takes me, I will never leave this work. I will never abandon this mission. Nor can you. If we are to honor those who came before us, and those still among us, we must match their sacrifice, their effort, with our own.”

Outside the chapel, crowds gathered in fewer numbers than the previous day, when President Obama gave a speech on race in America at the foot of the bridge.

The mood in the crowd was lighter and more celebratory Sunday, without the edginess that came with metal detectors, pat-downs and a Secret Service presence.

The Students Unite organization was among the first to cross the Edmund Pettus Bridge on Sunday morning, supported by the Freedom Foundation of Selma. They locked arms, they sang, they kneeled and prayed.

Lola Akingbade and Akiera Gilbert led a group of fellow students from Northeastern University in Boston as part of the Students Unite group.

“We see Selma not only as representing the past but a point of reference for the future,” Akingbade, 19, said. “We wanted to be involved . . . to have our voices heard.”

Gilbert said living in the country’s Northeast was sometimes like living in a bubble, with students somewhat “shielded from what’s happening in our country.”

Being in Selma “teaches us . . . a totally different perspective on racial issues,” Gilbert said.

Earlier Sunday, Selma officials paid tribute to the late President Lyndon B. Johnson for the Voting Rights Act. Luci Baines Johnson accepted an award on behalf of her father, saying that seeing him honored was meaningful, AP reported.

“You remember how deeply Daddy cared about social justice and how hard he worked to make it happen,” she told a crowd Sunday morning, according to the AP. Several hundred gave her a standing ovation, and some chanted, “L.B.J., L.B.J.!”